1. Field of the Invention
The present invention relates to a discharge gas treatment catalyst for use in removal of nitrogen oxide (NOx) and sulfur trioxide (SO3) from a gas discharged by a boiler or a similar apparatus.
2. Background Art
In order to remove nitrogen oxide (NOx) from contained in a discharge gas produced from a boiler, a gas turbine, a combustion furnace, etc., in practice, there has currently been employed an ammonia catalytic reduction method in which NOx is decomposed by use of ammonia (NH3), serving as a reducing agent, in the presence of a nitrogen oxide removal catalyst (hereinafter abbreviated as “an NOx removal catalyst”), to thereby form nontoxic nitrogen and water.
In many cases, boilers and similar apparatuses employ coal or fuel oil C, having a high sulfur content, as fuel. When such fuel is burnt, the discharge gas contains sulfur dioxide (SO2) at high concentration and sulfur trioxide (SO3). During treatment of the discharge gas, two reactions occur in parallel; i.e., NOx reduction for removal thereof and oxidation of SO2 to form sulfur trioxide (SO3). As a result, the discharge gas comes to have increased SO3 content. The thus-formed SO3 and remaining NH3, which has not reacted in the NOx reduction for removal thereof, are readily combined together at low temperature, thereby forming compounds such as acidic ammonium sulfate. These compounds, including acidic ammonium sulfate as well as SO3, corrode piping and the interior of an apparatus such as a heat exchanger disposed on the downstream side of the discharge gas treatment system. The corrosion causes plugging, partial clogging, etc., increasing pressure loss. Thus, in order to prevent pressure loss, countermeasures such as performance enhancement of a dust collector must be taken.
Japanese Patent Application Laid-Open (kokai) Nos. 10-249163 and 11-267459 disclose NOx removal catalysts formed of oxides (e.g., tungsten oxide and vanadium oxide-tungsten oxide) carried by titania, which catalysts exhibit excellent NOx removal performance and low SO2 oxidation performance (i.e., preventing oxidation of SO2 to SO3).
However, when the aforementioned catalysts are employed, about 0.1% of SO2 is oxidized to SO3. Thus, demand exists for complete suppression of oxidation of SO2 to SO3 occurring simultaneously with NOx removal reaction, and reduction of SO3 level of a discharge gas, and, for example, Japanese Patent Application Laid-Open (kokai) Nos. 10-249163 and 11-267459 disclose a variety of techniques to satisfy the demand. However, there have further been demanded effective reduction of SO3 and NOx contained in a discharge gas as well as further prevention of SO3 formation which would undesirably occur in catalytic reaction.